Modeling of structure formation in Langmuir films of nanoparticles

Transcription

1 A. Agod, A. Vincze, J. Kertész, M. Zrínyi, Z. Hórvölgyi: Simulation of aggregation in two dimension (in Hungarian), Proc. of Vegyészkonferencia 99 (Hungarian Technical Sciences Society of Transylvania), Kolozsvár (Cluj Napoca) Rumania (1999) A. Agod, A. Vincze, J. Kertész, M. Zrínyi, Z. Hórvölgyi, Connection between the aggregation mechanism and the cluster size distribution (in Hungarian), Proc. of Vegyészkonferencia 2000 (Hungarian Technical Sciences Society of Transylvania), Kolozsvár (Cluj Napoca) Rumania (2000) É. S. Nagy, Y. Li, A. Agod, K. Barkács, J. Pálinkás, N. M. Esmail, Z. Hórvölgyi: Flocculation and Rheological Investigation of Aqueous Kaolin Suspensions (in Hungarian), Proc. of Vegyészkonferencia 2001 (Hungarian Technical Sciences Society of Transylvania), Nagyvárad- Félixfürdő (Oradea-Felix Spa), Rumania (2001) lectures with abstract A. Vincze, A. Agod, J. Kertész, M. Zrínyi, Z. Hórvölgyi: Interfacial aggregation of spherical microparticles: computer simulations and real experiments, 3 rd European Conference on Computational Chemistry, Budapest, Hungary (2000) Abstr. O-17 (p. 41) A. Agod, M. Zrínyi, Z. Hórvölgyi, Two-Dimensional Growth of Patterns Composed of Fine Particles, International Conference for Physics Students, Dublin (2001) M. Vörös, P. Basa, A. Agod, M. Zrínyi, Z. Hórvölgyi: Analysis of the cluster size distribution of 2D aggregates 8 th Conference on Colloid Chemistry, Keszthely, Hungary, (2002) Abstr. OL-40 (p. 54) patent A. Agod, S. Beleznai, L. Jakab, L. Reich, P. Richter, Dielectric barrier discharge lamp, Date of filing: July 04, 2005, Patent No ATTILA PÉTER AGOD Modeling of structure formation in Langmuir films of nanoparticles Advisor: Dr. ZOLTÁN HÓRVÖLGYI Department of Physical Chemistry Budapest University of Technology and Economics 2006

2 lectures with abstract A. Agod, Gy. Tolnai, Z. Hórvölgyi: Compression of nanoparticulate arrays in a film balance: computer simulations, 8 th Conference on Colloid Chemistry, Keszthely, Hungary (2002) Abstr. P-1 (p. 61) A. Agod, Z. Hórvölgyi, P. Camp: Simulation of compression of nanoparticulate monolayers, TRACS-ACCESS-MINOS User Group Meeting, Barcelona, Spain (2003) Z. Hórvölgyi, A. Agod, E. Hild, E. Kálmán, Gy. Tolnai: Film balance and laser-optical investigations of silica nanoparticulate films formed at waterair interface, XVIIth Conference of the European Colloid and Interface Society, Florence, Italy (2003) Abstr. OR5/12 (p. 120) A. Agod: Computer simulation of preparation of nanostructured thin layers by wet colloid chemical method, First Conference of PhD Students at Faculty of Chemical Engineering, Periodica Polytechnica Ser. Chem. Eng. 47 (1) (2003) OTHER PUBLICATIONS papers M. Vörös, A. Agod, P. Basa, M. Zrínyi, Z. Hórvölgyi: Analysis of the cluster size distribution of 2D aggregates in terms of polydispersity, Progr. Colloid Polym. Sci. 125 (2004) A. Vincze, A. Agod, J. Kertész, M. Zrínyi and Z. Hórvölgyi: Aggregation kinetics in two-dimension: Real experiments and computer simulations, J. Chem. Phys. 114(1) (2001) Sz. Beleznai, G. Mihajlik, A. Agod, I. Maros, R. Juhasz, Zs. Németh, L. Jakab, P. Richter: High-efficiency dielectric barrier Xe discharge lamp: theoretical and experimental investigations, J. Phys. D: Appl. Phys. 39 (2006) lectures with proceedings A. Agod, A. Vincze, J. Kertész, M. Zrínyi, Z. Hórvölgyi: Modelling of interfacial aggregation phenomena (in Hungarian), Műszaki Kémiai Napok 99, Veszprém, Hungary (1999) Proc. 238A. 9

3 PUBLICATIONS CLOSELY RELATED TO THE THESIS INTRODUCTION papers Gy. Tolnai, A. Agod, M. Kabai-Faix, A. L. Kovács, J. J. Ramsden, Z. Hórvölgyi: Evidence for secondary minimum flocculation of Stöber silica nanoparticles at the air-water interface: film balance investigations and computer simulations, J. Phys. Chem. B 107 (2003) In the colloid chemical approach of nanotechnology, molecules and nanoparticles are the building blocks of the new materials. The macroscopic properties of these nanostructures are determined not only by the characteristics of the individual particles but also by their spatial A. Agod, Gy. Tolnai, N. Esmail, Z. Hórvölgyi, Compression of nanoparticulate arrays in a film balance: computer simulations, Progr. Colloid Polym. Sci. 125 (2004) A. Agod, E. Hild, E. Kálmán, A. L. Kovács, Gy. Tolnai, Z. Hórvölgyi: Contact angle determination of nanoparticles: real experiments and computer simulations, J. Adhesion 80 (10-1) (2004) S. Bordács, A. Agod, Z. Hórvölgyi: Compression of Langmuir films composed of fine particles: collapse mechanism and wettability, Langmuir 22 (16) (2006) order. Thus, if the mechanisms of the structure formation are known, their physical and chemical properties can be designed in a more systematic way. This is where computer simulations can support us: as opposed to the real world, a computer model makes it easy to move in the multidimensional space of input parameters, thus correlations between the measurable quantities and the microscopic mechanisms are much easier to reveal. Structuring a material on the nano-scale can provide new N. Nagy, A. Deák, Z. Hórvölgyi, M. Fried, A. Agod, I. Bársony: Ellipsometry of silica nanoparticulate LB films for the verification of the validity of EMA, Langmuir (2006), in press A. Agod, N. Nagy, Z. Hórvölgyi, Effect of polydispersity on the structure of particulate Langmuir films: computer simulations, submitted to Langmuir lectures with proceedings A. Agod, Gy. Tolnai, Z. Hórvölgyi: Simulation of film balance experiments and determination of the thickness of stabilizing water interlayer between nanoparticles (in Hungarian), Proc. of Vegyészkonferencia 2002 (Hungarian Technical Sciences Society of Transylvania), Kolozsvár (Cluj Napoca) Rumania (2002) 20-25a Z. Hórvölgyi, A. Agod, E. Hild, E. Kálmán, Gy. Tolnai: Determination of the contact angle of nanoparticles in a film balance (in Hungarian), Proc. of Vegyészkonferencia 2003 (Hungarian Technical Sciences Society of Transylvania), Kolozsvár (Cluj Napoca) Rumania (2003) physical or chemical characteristics, e.g. modulating the dielectric permittivity on the length scale comparable to the wavelength of light results in photonic crystals or antireflective coatings. The electronic band structure of nanometer sized particles may differ from the macroscopic bulk characteristics due to size-quantized effects. The relatively simple and inexpensive Langmuir-Blodgett (LB) technique allows us to exploit these novel properties on the macroscopic scale. By fabricating multilayers from the films of various types of nanoparticles, optical, catalytic, electronic, magnetic or even mechanical properties of the materials can be combined. To gain a better understanding of the structure formation of Langmuir-films, we developed a computer simulation that permits the modeling of film balance experiments of partially wettable, spherical S. Bordács, A. Agod, Z. Hórvölgyi: Investigation of the structure of microparticulate monolayers in a Wilhelmy film balance (in Hungarian), Proc. of Műszaki Kémiai Napok 2005, Veszprém, Hungary (2005) 231 nanoparticles. Using this model we interpreted the collapse mechanisms of particulate films, assessed the contact angle of Stöber silica particles 8 1

4 and revealed the relationship between the size distribution and the APPLICATIONS evolving structure. We proposed a correction method for the determination of interparticle potentials from surface pressure vs. area isotherms and investigated the structure of multilayers fabricated by LB technique. To strengthen the conclusions of the computational work, we carried out real film balance experiments on the model system of microparticles having partial wettability. Using the LB technique, photonic crystals, nanolithographic masks, planar waveguides and other nanostructured coatings can be fabricated from the Langmuir films of nanoparticles. The quality of the thin film on the substrate is determined by the structure of the particulate monolayer at the liquid-fluid interface. The demonstrated computer simulation permits the investigation of particulate monolayers of arbitrary size METHODS distribution, contact angle and particle interactions, thus it can be used as a design tool for tailoring nanostructures. (Fig. 4) In real systems, a few Molecular dynamics computer model was developed to reproduce the nanometer thick liquid film can be present between the particles due to macroscopic behaviour of real systems and thus reveal the their repulsion; the polydispersity is often in the range of 10-20%, and the microscopic mechanisms on the length scale of the individual particles. A two-liquid Wilhelmy film balance was built, and monolayers of easily visualized, surface modified glass microparticles were compressed at structure of the monolayer is hexatic, at the best. Our computer simulation can account for the effects of these imperfections and thus facilitate the optimization of the structure of LB thin films. various liquid-fluid interfaces. Our conclusions were drawn from the comparison of real and simulated experiments on Stöber silica systems, and the argument was validated by the results of experimental work on microparticulate model systems. RESULTS 1. Molecular dynamics computer simulation has been developed to Figure 4: Simulated structure of an LB multilayer. The 6 sublayers were taken at 10 mn/m surface pressure and consist of averagely 44 nm diameter Stöber silica particles. model the compression of Langmuir films of spherical nanoparticles. Arbitrary particle size distribution, contact angle, compression rate and central pair interaction can be set in the model. Since the particles are allowed to move perpendicular to the liquid-fluid 2 7

5 7. We developed a computer model to investigate the structure of interface, the structure formation can be investigated after the onset multilayers fabricated by LB technique. The inputs of this model were the monolayer structures gained from the simulation of Langmuir films of the same particulate sample. The results indicate that the porosity as a function of distance from the substrate has local maxima. The values of these maxima are continuously decreasing, and the variance of the distance of particles belonging to the same layer levels off after 4-5 layers. (Fig. 3) By means of this model we were able to investigate the structure of multilayers fabricated from realistic Langmuir films, i.e. monolayers of polydisperse particles not only in close packed, perfect hexagonal order. of the collapse. 2. By means of the simulation we proved, that at a given surface coverage a real particulate film has higher surface pressure than an idealistic monodisperse, hexagonally packed system due to the uniaxial compression of real films and the polydispersity of nanoparticles. Consequently, the usual assumption in the determination of interparticle potentials from the surface pressure vs. area isotherms (i.e. monodisperse particles in perfect hexagonal order), leads to a systematic overestimation of the characteristic length of the interaction. Based on the results of the simulation we proposed a correction method to improve the traditional way of space filling z (nm) layers Figure 3: Space filling of a simulated multilayer structure and its sublayers as a function of distance from the substrate. The sublayers are outputs of the simulation of Langmuir films and consist of 44 nm diameter Stöber silica particles sum determining the interparticle potentials. 3. A Langmuir film consisting of narrow size distribution, spherical particles can have two types of collapse mechanism: buckling, where the whole layer folds and wrinkles, or expelling, where particles are displaced from the interface. According to the congruent results of the experiments on microparticles and the simulation, the mechanism of collapse is primarily determined by the wettability of particles. Expelling mechanism is expected if the particles are well wetted by the subphase, whereas the layer folds during collapse for less wettable particles. In terms of contact angles, the mechanism change is between 40 and 50 degrees. In the case of expelling collapse mechanism the increasing polydispersity of particles results in lower collapse pressure. 6 3

6 4. We showed experimentally that the contact angle determination 6. The particulate Langmuir films are continuously restructuring in based on integrating the surface pressure vs. area isotherms considerably overestimates the contact angle of particles. Instead of course of the uniaxial compression. According to the results of the computer simulation the adjacent particles usually shift in the same that, we proposed a new method based on the results of the direction, therefore the local structure is mostly preserved. Units computer model. At given particle-particle interactions and size consisting of numerous particles move collectively to follow the distribution the collapse pressure of a Langmuir film is a monotonous function of the contact angle and the simulation can reveal this constraint dictated by the barrier. (Fig 2.) Because of the uniaxial compression and the stiffness of the particulate layer, the surface correlation. (Fig. 1) That contact angle will be accepted at which the pressure is not a scalar field, its value depends on the direction angle. simulation provides the experimentally measured collapse pressure. In systems of smaller polydispersity bigger units move in course of contact angle (deg) restructuring, thus the ratio of eigenvalues of the local pressure tensor fluctuates more during compression. According to the simulation, the narrower the size distribution the more significant the difference between the values of surface pressure parallel and perpendicular to the barrier. Π c (mn/m) Figure 1: The contact angle of particles can be estimated by comparing the experimental (red) and the simulated (orange) collapse pressures (Πc). 5. The correlation between the size distribution of particles and several structural parameters (decay length of pair-correlation function, global orientational order parameter, mean and standard deviation of number of neighbours) has been explored. We found that domainstructured films can be formed only below 7-8% polydispersity and showed how the computer model can support the design of nanostructures. By means of the simulation one can find the ideal size distribution - surface pressure combination to fabricate Langmuir- Figure 2: Simulated restructuring of 44 nm diameter Stöber silica particles. The lines ending in the center of particles show the directions where particles arrived from. The direction of compression is vertical in the figure. Blodgett films of desired surface coverage and order. 4 5